ouantitative electron.microprobe analysis of alkali silicate

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ouantitative electron.microprobe analysis of alkali silicate
323
The CanadianM ineralogist
Vol. 33,W.3%-332 (1995)
OUANTITATIVE
ELECTRON.MICROPROBE
ANALYSIS
OFALKALISILICATE
GLASSES:
A REVIEWAND USERGUIDE
JOHN G. SPRAY
Departrnent
of Geology,University
of NewBrw"rwickFredericton,
NewBrw"ywick
E3B 5A3
DAVID A. RAE
ElectronMicroscopy
Unit,University
E3B6El
of NewBrmswicbFredtricton,NewBrunswick
ABSTRA T
The mobilizationof alkali metalsin alkali silicateglassesandcertainmineralsduring electron-microprobe
analysisis a result
of beam-inducedheating and charging effects within the sample.The following procedwesare recommendedin order to
minimize theseeffects.(1) Totat beam-powershouldbe lessthan 100pW to reduceheatingwithin the inadiatedvolume;this is
best achievedby decreasingthe beamcurrentand by using a cryogenicstage,though the latter may be impracticalfor routine
analysis.(2) Samplechargingcan be minimized by usrnglower beam-power,but higher voltagesmay be requiredto displace
accumulatingelectons to deeperlevels within the sample.(3) Heating and charging effects can be reducedsignificantly if
defocusedbeam or raster scan-modesare used. (4) Sampleconductivity can be improved by applying double carboncoats,
by coating both sides of the sample,and by using conductiveslide-mounts(i.e., copper insteadof glass). (5) Count-times
shouldbe shorGn€dto minimize the rate and sat€a16f alksli migration, while maintainingthe statisticalvalidity of the alata(6) Conectionsfor alkali migration shouldbe doneprior to ZAF corrections.The low power requirementsand high efficiency
of the Si(Li) detector clearly favor [email protected]) over wavelength-dispenionspectroqetry (WDS) for the
quantitativeanalysisof alkali silicate glasses.
Keywords: alkali silicate glass,electron-microprobeanalysis,energy-dispersionspectrometry,wavelength-dispersionspectrometry, alkali-metalmobility, space-charge
layer, electromigration.
SoraMens
Ia mobilisationdesalcalinsdansles verressilicatdsd alcalinset danscertainsmin6rauxpendantuneanalysei la microsonde
6lectroniqueest le resultat du r6chauffementd'un 6chantillonpar le faisceauet de I'accumulationdes chargesintemes. k
protocolesuivantserti minimiser ceseffets. (1) La puissancetotale du faisceaudevraitOtreinf6rieurei 100pW afin de rdduire
la chaleurdu volumeirradi6; on doit doncdiminuer le courantdu faisceauet utiliser un porte4chantillon cryog6nique,quolque
cettedemidremesurepourrait s'av6rerincommodedansle contexted'analysesroutinibres.(2) L'accumulationdes chargesest
minimis6een r6duisantla puissancedu faisceau,quoiqu'unpotentielplus 6lev6pourrait s'av6rerndcessairepour deplacerlqs
6lectons accumul6svers un niveau plus profond dans l'6chantil1on.(3) Les effets dus au chauffementet I I'accumulation
deschargessont sensiblementr6duils en utilisant un faisceaud6focalis6ou un modede pr6lbvementdesdonndespar balayage.
(4) Ia conductivitdde l'6chantillonpeut 6tre arn6lior6een appliquantune doublecouchede carboneou une couchedes deux
c6tdsde ldchantillon, et en adoptantune lame i conductivit6plus 6lev6e,faite par exemplede cuiwe au lieu de verre. (5) Le
tempsde comptagedevftdt 0[e raccourciafin de minimiser le taux et la port6ede la migration dss alsalins,tout en maif,tenant
la validitd statistiquedesdonn6es.(6) ks correctionsvisant d compenserpour la migration desalcalinsdevrait Otreeffectudes
avant la correction pour le nombre atomique,I'absorptionet la fluorescence.Il est dvident que les faibles exigeancesen
puissanceet I'efficacit6 61ev6ed'un d6tecteurSi(Li) favorisentune analysequantitativede verressiliceux richesen alcalinspar
dispersiond'6nergieplut6t quepar dispersionde longueursd'onde.
(Iraduit par la R6daction)
Mots-cl6s:verre siliceux e akalins, analysepar microsonde6lectonique, dispersiond'6nergie,dispersionde longueursd'onde,
mobilit6 dessls:lins, couchesurcharg6e,6lectomigration.
3V1
TT{E CANADIAN
MINERALOGIST
Ixrnopuctrox
Silicate glassesare an importantgroup of materials,
of inlerest to both the Earth scientist and indusry.
For naturally occurring aluminosilicateglasses,much
can be learned about the geological conditions of
their forrnation if accurateand precisecompositional
data can be obtained.In commercial applications,it
may be necessaryto analyze synthetic glasses on
a routine basis for purposesof quality conhol. Much
ofthe work on natural glassesrequiresthe analysisof
microscopicvolumes:zonesof intercumulusmaterial
in igneousrocks, fhin rinds of glass on pillow lavas,
and veins of friction-generated"melto' (pseudotachylyte) generatedalong slip surfaces.Many run
products of .the experimental petrologist comprise
small volumes of synthesizedglass (e.g., Beard &
Lofgren 1991). The electron nr,icroprobe(Castaing
1951) is an ideal tool with which to analyze these
materials,but it is widely recognizedthat the alkali
metals (particularly Na) can diffrrse out of or into the
analyzedvolume (e.9., Sweatman& Long 1969,
Goldsteiner aL 1992,Reed1993).This effect is dependent on a number of factors, including the SiO2 and
H2O contents of the analyzed material, and the
dissipationof power aroundthe excitationvolume. As
a consequence,
it is easyto collect incorrect analytical
data and accurateanalysiscan be difftcult. This work
reviews the situation regarding the electron-microprobe analysisof silicate glasses,presentsmodels for
the causeof ionic mobility, and suggestsprocedures
for the minimization of beam-inducedlossesand gains
in alkaii metals.
Nerunar, Gussns
Many natural glasses are of volcanic origin
Clable 1). The composition of a quenchedigneous
product can be informative; basic glassescan be less
fractionatedthan associatedcrystalline rocks, and so
indicate more about source maglmus(e,g., Natland
1991). Glassescan also be diagnosticof deformation
processesat high and ultrahigh stain-rates, such as
seismic slip (e.9., with the formation of pseudotachytyte: Spray 1987, 1993) and ."1eef1s impact
(e.9., with the generationof tektites: Koeberl 1986).
The high-speed atmospheric entry of meteors can
also induce glassy fusion-induced crusts on their
surfaces due to air-projectile frictional heating
(McSween 1987). Diaplectic glass is produced by
the passageof a shock wave through a rc& (e.g.,
Bischoff & Stiiffler 1992), with the ransformarion
in the solid state without melting. Certain
metamict minerals (Table 1) are rendered glassy
by o-particle-induced structural damage during
the decay of unstable isotopes (e.g., of U, Th).
However, none of these minerals contain significant
alkalis.
[email protected]
ONdlm
Plt(fistone
Tadylyle
P€le's bah/tem
Glassypyrelasdc marertal
rclcadc ald
[email protected]
Plaretary regotitbs (eg" m)
Meteorite shock veirs
Tekdtes
Dt plscdtes (eg, narkelydte)
Ishatolisrlter
lryacr-indued
(supercooled
or diaplesdc)
Pmilotaclylyte
Furion 6usts on mstootitgs
ficdo-tndrced
dd [email protected]
Meanict minerals (e.& 8[adtc,
nonsdtg, thodl€, drcon)
iradiated
t ds Foduced by UglinhC stdks in mnd (fulgurtte)
SvNmsnc GI-A,ssEs
Syntheticinorganicglasses(fable 2) can be classified into four groups.Network-moffied SiO2glasses
are the mo$t important in terms of commercial
[email protected],Zarzyck 1991).Halide,
chalcogenideand metallic glassesare relatively recent
TAII.B 1 MAN SY}IIIBTIC INOBCAMC (IASSES
TYpc
C€mpm€nl(o
Odde glas
sio,
(usallywfth
notwort-modlien)
Chedel redstasce"
thermal sho{&rgistae
(e.g; borosiltere - pyr*)
&cr.
As dngle componeDlddeq
the{g ore aons!ily of
sdenfnc bfrest
Ealtdo glas6
BeFr ZrF& AIF9
I{authdde)Fe
ZnFlPbFb Bafr
7-frtet.
(Lo" potEDrlal
opd€l fibr6)
B;@[ed opdcal
tmsndldm ltroperdg
Chalcogeltde
glas
Group VI elsm€ofi
combhed vith
Grcups IV and V
elgmsl8
Idared opdel tmndsdm
and electtcal mitc&ing
MetalUcglas
Melal-retalloidr
M€tal-relal
Very lw magrctic lmos,
Hgh nec.hadol strmglh ud
hardness,ldgh c.h€dcal
mnodon redstance md
radladon redr&ace
Plo!
Geq
e metallold: e.C. B, C' Si B Ce
TTIE ELECTRON-MICROPROBE
ANALYS$ OF GLASS
325
developments,and have various advanced-technology lower beam-current requirement of the ED
applications.Network modifiers in the SiO, glasses specffometermeansthat EDS is the bettermethodfor
iuclude Mg, 84 Ca, Li, K and Na. Qompositional determiningthe major-elementcompositionof glasses
delerminationsof manufachred silicate glass at the (bearing in mind that EDS power requirementsare
microscopic scale are not usually a routine require- largely dependenton the distanceof the detectorfrom
ment. This is because the amounts and types of the areaanalyzedand the geomefiryof the collimator
ingredientsare predeterminedat the designstage,and and silicon detector).
so ttre composition is known and controlled.
Furthermore,the product is normally homogeneous.
TrrEEFFFcTor Er.rcrnou BoNmapoNmvr
However,electon-microprobeanalysiscanbe usedfor
oN ALKALTSrrcars Gressss
rapid quality-conhol and for assessmentof compositional homogeneity.In theseinstancesn
the imFortance
How can mobilization of alkali metals be recogof Na as a network-modiSing cation can lead to nized?(l) Alkali-metal count-ratemay shift at constant
problemswith analyticalaccunrcy.
voltageand current.(2) Loss may be indicatedby low
analytical totals; however, if the glass containsH or
EDS vm.susWDS ANeryss
halogens,low overall totals can be due to HrO content
(e.9.,Stolper1982)or halogenloss(e.9.,Starmeret al.
X-rays emitted from the samFle surface can be 1993). Gains are more difficult to detectbecausethe
collectedby either an energy-dispersionspectrometer analyticaltotal may not be noticeablyhigh. Gainscan
spectometers(WDS). occur where extended count-times, higher [email protected])or wavelength-dispersion
The fust commercial electron microprobe that currents, or thinner conductive coatings are used,
appearedin 1958 deployedonly WDS spectometers, especiallyif the glasshas a low thermal conductivity
and it was not until the lale 1960sthat the solirl-state @orom & Hanneman1967). However, nlkali-metal
lithium-drifted silicon [i.e., Si(Li)] detector was gainsoccurlesscommonlythanalkali-pe1allesgss.
developedto eventually facilitate quantitative EDS
If loss has occurred,a map of Na disfibution viill
[email protected] et aL 1968,Reed 1993).The two show a sodium'hole'that approximatesthe excitation
t)?es of specftometerdiffer in a numberef important diameterof the elecfton beam (Fig. 1a). A line scan
respects(Table 3) that are pertinentto the analysisof acrosssuch a hole has a pit-like [email protected] lb). The
alkali silicate glasses.The WDS spectometer has rate of Na loss varies with time. An initially steep
higher resolution and better detection-limits(10 ppm declinein countrate is typically followed by a reduced
for optimum operatingconditions)than doesthe EDS but relativd steady-statecount-rate.With loss of Na
specfrometer.However, the detector efficieucy for from the excitation volume, the count rates of the
WDS is significantly poorer than that of bDS. lsmaining elements increase. This occurs because
Consequently,whereasbotl typesof spectrometercan the electronmicroprobemeasuresthe massfraction of
be operatedat comparablevoltage,the WDS requires elementspresentin ttre interaction volume, so that a
higher beam-currentsto achieve an adequa0eX-ray loss of one or more elementsis compensatedby an
count-rate (Table 3). The higher power required for apparentgain in those
This is illustratedin
WDS results in higher temperatures and greater Figure lc, where the relative decreasein Na counts
chargingwithin the sample,especiallywherea focused over time is mirrored by a relative increase in Si
beamis used,and this promotesdiffirsion of the alkali counts.Varsbneyaet al. (1966) documenteda similar
metalswithin the silicate glass.
effect in a KrO-SrO-SiO2 glassowing to K loss. If a
The advent of improved EDS systemsmeansthat gain has occurred,a map of Na distibution will show
fully quantitative major-element analysis is now a Na build-up (Frg. 1d). A line scan across such a
routine (e.9., Statham& Nashashibi1988).Thus, the build-up has a mound-like shape @9. 1e). If Na
increaseswithin the analyzedvolume, fhe counl rates
of the remaining elementsdecrease,suchthat there is
TABI.B 3. CSARAC'IERFTICS OF M6 VRSUS WDII
an apparentreductionin their content(Fig. 10. Boror
ATiIALITICAL TECMtrQI'A
& Haoneman (1967) showed that this occurs for
K2O-SiO2and Na2G-FeO-SiO2glassesunder cerlain
wDs
operatingconditions.
Steady-stateconditions eventually lend to prevail
for alkali-metalloss, althoughfor long-durationcount15.30
ur-30
lpical voltage range ftV)
times (severalminutes), initial lossesmay be superVtable surert nnge (rA)
0r5-5
s-50
Power rarge (pW)
3.75- 100
75 - 1000
sededby gains. If alkali-metal gains occur on initial
Resolution (eV)
<u0
<10
bombardment,
they can persist or become highly
Optirum detectlon limits
O.L'ntVo
<100ppm
of the mobility
Detec{or
solld state
gasphase erraticwith time. The time-dependency
-1fr)
Detecton efidency (7d
*S
of alkali metals has important implications regarding
fhe count-timesthat areusedfor analvsis.
326
TIIE CANADIAN MINERALOGIST
(d)
/||lm
{nm
(e)
(b)
L
h
I
*K
asl-
(c)
(D
I
I
1.0
1.0
l(s)
(8)
rt(8)
Ha. 1. (a) Part plan view of a polishedthin sectionof an alkali silicate glass showing
"hole" where Na is absent.(b) Section
Na distribution. Note electron-beam-induced
'\rell".
line-scanfor Na for samesampleasabove(cts = counts).Note presenceof Na
(t) of X-ray intensities(Q normalizedto zerotime for Na andSi.
(c) Time-dependence
Note Na loss andSi gain. (d),Planview asfor (a) above,but note zoneofNa increase.
(e) Sectionline-scanfor Na for sane sample(d) asabove.Note Na 'lnound". (f) Timedependenceasfor (c) opposite,but note overall Na gain and Si loss, aswell asinitial
reversalin tends.
The degreeof elkali-metalmigration is also dependent on the mode of analysis.Point (focused-beam)
analysis results in greater migration, whereasdefocused beam and raster scanning have less extreme
effects.Table4 showsthe resultsof analyzingthe same
point or area four times using poing defocusedand
raster scan-modeson basaltic glass VG2 (Jarosewich
1975).The meanNa2Ovaluefrom the point analysisis
the lowest and this method shows a more consistent
depletion in Na for successiveanalysis of the same
spot.Both defocusedandraster-scanmodesproducean
acceptable mean result, which is higher tlan the
averagepoint analysisand closerto that of the quoted
internationalstandardvalue. Furthermore.there is no
decreasein Na2Ocontentwith successiveanalysiswith
defocusedandrasterscan-modes.
The next slep is to ty to understandhow alkalimetal migration occurs, so that the problem can be
avoidedor minimized.
Ceusrs oF MoBIurY or Alr.lu ME"rALs
DuRn{cEr.rcrnoN Bommomlr
A number of mechanismshave been proposedto
accountfor alkali-metal loss and gain during elecfron
bombardmentof alkali silicate glasses.Theseinclude
sample volatilization (Baird & Tnnger 1966) and
electromigration (e.g., Graham et al. 1984, Cazaux
327
TTIE ELECTRON-MCROPROBEANALYSIS OF GI.ASS
TABIT 4. PS ANALYSES.OFMSAL]IC GI.AIISSTAT{DAIDVE
Fm. Nr uS!NGDIFFBENT BEAM-MODEI
beam mode
I
2
3
4
foo$ed
(D - 1p-)
defocrsed
raster
L70
2.47
22l
2i6
274
2.80
273
[email protected]
?^71
253
269
LP
L72
2.69
@ - z0pn)
Mean 245
pm)
1- r,oOO
order of 1000'C or more may arisefor focusedbeams
(1 pm in diameler)in uncoatedsamples.Analyzing a
s,ampleof low thermal conductivify (1.e.,a glass) in
an uncoatedstate can lead to beam-inducedsurface
damage,manifest as pits and craters,some of which
developelevatedrims @orom & tla:meman 1967).It
a coating medium is used (usually carbon for geological materials),the temperaturerise can be reduced
by as much as an order of magnitude(1e., down to
approximately100'C).Vassamillet& Caldwell(1969)
equaled the energy dissipation of the beam to an
equilibrium temperaturefor the periphery of the iradiatedvolume given by
T-t=
Standardvalue2.62
Cr,I2t*r)
(t)
where T is the rise in lemperatureat the peripheryof
the inadiated volume, \ is the ambient temperature
of the sample,W is power in watts, ft is the thermal
conductivity of the sample(W m-l oc-l), and r is the
1986). Autefage & Couderc (1980) suggestedthat radius of irradiated area in meters.For a K"O-SiO"
alkali-metal mobilization occurs in two stages: (1) glass,Vassamillet& Caldwell (1969) foundthat T"
thermally activatedbreakingof Na-O bondsandinitial (the oitical lemperaturefor K mobilization)is approxionic diffrrsion, and (2) elecfiomigration of free Na+ imately equalto 85oC,andis likely to be lessthanthis
ions to the zone of electron build-up (space-charge for Na becauseof its greaterionic mobility.
layer).
Figure2 showslemperatureincreaseasafunction of
power and beam radius using a thermal conductivity
oc-l appropriatefor silicate
Beam-induced thermal ffi as
ft) value of 1 W m-l
glasses(Ashby& Jones1986,p. 151).Position3 indiCast?ing(1951) and Friskney & Haworrh (1967) catesa value for focused-beamoperationsuitablefor
have calculated that for operating condifions of WDS analysis (1 pra at 300 pW). Position z also
25-30 kV and 100 nA, temperatureincreasesin the correspondsto the proposedT" of Vassamillet&
rOperating
15kY 2J nA.
conditioos:
.001
.01
0.1
1
10
f (rrm)
Ftc.2. Electron-beampower rersasbeamradiusandresultanttemperatureincreasesatthe
periphery of the irradiated volume using the equation of Vassamillet & Caldwell
(1969). Point 3 conespondsto t)?ical WDS conditions of analysis (see Table 4),
as well as the temperatureof onset of diffusion ef slkali metals in silicate glasses
(T" * 85'C). Defocusedbeamradii shownshaded.Powerrequirementsfor WDS and
EDS analysisareindicatedon right.
328
TTIE CANADIAN MINERALOGIST
Caldwell (1969), which shows that those values
parallel to the isotherms and to the left of z would
facilitate alkali-metal migration e85'C). Note that
typical valuesof r for a defocused$sam (i.e., yrelding
analyzedareasof 10-20 pm diameter)resultin T being
muchlessthan T", suchthat bond disruptionanddiffrrsion are avertedfor both EDS and WDS operation.
Overall, it can be seenthat the likelihood of mobiliz.ing Na is significantly reducedwith EDS compared
to WDS. In fact, for WDS analysis,it seemsdifficult
not to mobiliz" 116e
alkali metalsin silicate glassesif a
focusedbeam is used. Use of low power with WDS
necessitates
long count-times,which are also counlgrproductive.
elemental form (inducing a color changeknown as
o'electronbrowning"), or to bond to new oxygenions.
A concomitantdecreasein Na X-ray intensities was
noted, and the analyzedvolume becarnedepletedin
this metal. Movement of Na ions down to the charge
layer resultsin oxygenbeing releasedfrom the upper
part of the samplewhere, on losing elecftonsto the
elecfodeoit escapesfrom the surfaceas [email protected] 4).
The Lineweaver mechanism has been widely
accepted,but it is not applicable in all situations.
Borom & Hanneman(1961 found that under certain
conditions,an increasein alkali-metalX-ray inlensities
can occur, with a correspondingdecreasein the intensities for the remainingelements.This is characteristic
of more intense beams and thinner conductive
coatings,especiallywherelessstablevarietiesof glass
are being analyzed.They atfibuted this to increased
thermaleffectsand to destruction(vaporization)of the
conductivecoating.This allows electronsto accumulate at the samplesurface,suchthat the mobile Na ions
are athacted toward the more negatively charged
surface.
Both the Lineweaver (1963) and Borom &
Hanneman(1967) models can be reconciled if the
space-charge
layer canmovewithin the sample.Estour
(1971) showed that the rate of decay in the X-ray
countsfor Na could be minimized if higher accelerating voltagesareused(i.e.,>20kY). This wasattributed
to the accumulationof elecfronsat deeperlevels in
the sampleowing to their greaterkinetic energy,such
that the near-surfaceNa ions are effiectivelyshielded
from the relocated more remote space-chmgelayer.
Thesparc-ch,argelayer
An important cotrcapt in our uuderstanding of
electromigrationis that of the oospace-charge
layet'',
which is purportedto developwherethe elecfrongain
of the sample exceeds tle electron dissipation.
1yl"1p1 alkali-metalions arelost from or gainedin the
inadiated volume dependson the size and location of
the space-charge
layer within the sample.
Lineweaver(1963),in his studyof NarO-SiO2glass
cathode-raytubes,showedthat oxygenis releasedfrom
glass surfaces{uring electronrastering,and proposed
that insident elecftonsaccumulateat somefin:itedepth
within the sampleto forrn a oonegative-charge
region"
(Ftg. 3). Owing to this electon build-up, Na ions
decouple from their oxygen (unbridged) ions and
migrateto the negativezone,to be eitherneuftalizedto
electron
beampit
o2
c"
II
+
tI
f"
*{f
Na"
Na*
-Na*
{
/
/**
Flc. 3. Cross-sectionof polished thin section of an alkali silicate glass showing the
formation of a "space-chargelayet'' comprising accumulated electrons within
the sample.Na-O bondsare broken, then Na cationsmigrate to the negativechmge
layer, wherethey are neutrlized.
329
TIIE ELECTRON-MICROPROBE
ANALYSIS OF GLASS
(a)
(c)
(b)
O2
Or
@ *^
r Sl
Q brldglngoxygen
orygen
O nonbrldglng
Flc. 4. Linelveaver's model for alkali-me€l loss: (a) prebombardmentstructure of
NarO-SiO2glass(simFlified); (b) Na-O bond breaking,downwardmigrarion of Na*
and upwardmovementof O! and (c) final (immediatelyposibombmdment)modified
stucture, with neutalized Na atoms and 02 evolving from the sample surface.
Modified after Lineweaver(1963).
Howeveroa higher voltagesipificantly reducesX-ray
generation,suchthat Estour(1971)had to increasethe
probecurrent(to 20 nA) in orderto generatesuffrcient
X-rays for the less efficient crystal spectrometer
100
$fDS). By doing so,he increasedlemperatures,andso
Na lossrecurred.Goodherv& Gulley (1974)confirmed
Estour'sfindings and showedthat probe currentstwo
ordersof magnitudesmaller could be usedwith EDS,
andNamobility couldbe virtually eliminated.Thebest
results were obtained at 30 kV and 0,2 nA (Fig. 5).
Goodhew (1975) subsequentlyshowed that if each
analysiswerecarriedout at a freshpoint on the sample,
the Na decayrale could be reducedbelow 0.5Voof the
counl rate. This would constituteonly l-2Vo loss in a
total count over a few hundredseconds.
o
o
Compositional depenlence of
allali -metal rnigration
01234567
Tlmslnmlnut€
Ftc. 5. Decay of NaKo count rate during EDS analysisfor
a variety of acceleratingvoltages at a beam cunent of
0.2 nA (after Goodhew& Gulley 1974).laad Io are the
X-ray intensities (counts) normalize<lto 100 at times
t = 0 + t andt = 0, respectively.
The susceptibility of glassesto beam-induced
mobilization ef alkali metalsis also dependenton the
chemical composition of the glass as well as on
the analytical operating conditions. Highly siliceous
glassesshow high susceptibility to alkali mobility,
whereasmorc basic Qow-SiO) materialdoesnot. The
forrner are desipated the so-calledunstable glasses
Q.ar.zyck:-1991) which, geologically, would include
rhyolitic glass. The stable glassesinclude those of
mafic composition.The reasonfor the variable alkalimetal response to electron bombardment remains
unclear,but it may dependon the degreeof polymerization of the SiOaand AlO4 tetahedrn.I high degps
TIIE CANADIAN MINERALOGIST
of polymerization involves a more open network of
tetahedra that allows diffrrsion of alkali metal cations
to occur with greaterease.Low polymerizationresults
in a moredsnselypackedsfucture ofthe glass,through
which it is more difficult for the alkali metals to
diffrrse. Depolymerizationis causedby, for example,
the presenceof Fe and Mg breaking the SiO2 chains
into shorterunits. As well as effectively blocking the
movement of alkali ions, these network-modifying
elementsincreaseelectricalandthermalconductivities
and hencereducethermal and charging effects. Most
geologically important glasses are aluminosilicates,
where the presenceof Al also acts to reduce alkali
volatility. The compositional dependenceof alkali
mobility must be borne in mind when selecting
operating conditions. tligh-SiO2, high-alkali-metal,
low-transition-metalglassesare particular diffrcult to
aralyze quantitatively.
Mwnnznqc Axeu MrcnenoN rN Gr.AssFs
[a e1d6p19minimizg alkali migration,it is necessary
to minimize both thermal effects and the developmenl
of the space-chargelayer. These two effects are
related. The energy required to break the Na(K)-O
bond is related to the temperaturegeneratedduring
powerdissipation.Alkali migrationis dependenton the
size of the space-charge
layer, which is controlled by
beampower and by the conductivitiesof both coating
and sample.The location of the space-charge
layer can
be confrolledby varying the beamvoltageandthe total
count-t'me.
Finally, it is noted that alkali migration is not
resfrictedto glassymaterials.Negativechargingof the
samplesurfaceand concomitantNa and K fluctuations
have beennoted amongstother minerals,in sepiolite
@utt & Vigers 1977), autunite(Grahamet aI. 1984),
feldspars (Ribbe & Smith 1966), feldspathoids
@rousseet al. 1969),jadeite and scapolite(Autefage
1980). In all cases,the minerals were analyzedby
WDS, and maximum mobility occurred when a
focusedbeam (i.e., approximately1 pm. in diameter)
wasused.
The following guidelines will help to optimize
analytical conditions for the quantitative analysis of
alkali silicateglasses:
(l) Power the temperature rise in the irradiated
volume within the sample is stongly controlled by
beampower.Reductionin kV is limited by the requirementfor 2-3 x overvoltageratio (e.9.,Fe 6.4 keV x 2.5
= 16 kV). Reducing the cunent will reduce thermal
effects, but this also lowers the intensity of X rays
generated, such that WDS analysis becomes impractical. Reduction of the beam .current is more
feasiblewith EDS.
Q) Relocation of the space-chargelayen the spacechargelayer may be moved deeperia fte sampleb]
higher accelerating
voltages(i.e.,30 kV). This mini-
mizesatfractionof alkalisfrom highel lsysls within the
irradiated volume. If beam power is to be kept low,
even lower currentswill have to be used and, again,
this favors EDS over WDS. It should be noted that
ZAF correctionsmay be in error at high kV.
(3) Initial tempera.tareof the sample: the beaminduced rise in temperatureCI) within the irradiated
volume may be greatly reduced by the ambient
temperature(t") of tle sample[seeequation(1)]. If a
cold stagecooledby a cryo-unit is used,about-140o
can be attained at tle stage. However, analysis
would become extremely time-consuming.Using a
conductive sample-mount(1e., copper) also helps to
improve heatremovalfrom the atalyzedarea.
(4) Beam.mode: rf.point analysisis not requird use
defocused(WDS or EDS) or raster scan-modes(EDS
only). Suchscanmodesinvolve a larger beam-area,so
that the power per unit area and hence temperature
increaseis reducedsipificantly. Rasteringis difftcult
with WDS because of more precise geometrical
requirements,but high-speedvdde-areaanalysistechniques have been developedwhereby the sample is
moved physically beneath a static lsam (a.9., Ono
et al. L985).
(5) Conduaivlry: with insulating materials,problems
of low thermalandelecfiical conductivitiesof samples
can lead to adversedevelopmentof the space-charge
layer. Th:is situation can be improved by thicker
(double)carboncoatsand by coatingboth sidesofthe
sample. heparation of ulfrathin samFlesglued onto
conducting (e.9., copper) mounts also may help to
alleviate development of the space-chargelayer.
However,too thick a coatingcancausemoreproblems
thanit alleviatesbecauseit increasesabsorptionofboth
incomingelectronsandoutgoingX-rays andsoreduces
the observedX-ray intensities.Furthermore,to obtaina
consistentthicknessof the coating, it is important to
ensurethat the standardsarc coatedat the sametime as
the samplesto be analyzedor, if coatedseparately,by
useof thicknessmonitor (Greaves1970).
(6) Coant-time:becauseof the time-dependentnature
of alkali mobility, reductionof the count-timecanhelp
to reducemigration. A careful balancemust be struck
between shorter count-times and accumulation of
adequate counts. Reduced coutrt-times are more
feasiblewith EDS becauseof the greaterefficiency of
Si(-i) detectors. If only two WD (or even three)
spgcfrometersare available,the samplemust be irradiatedfor a long period to collect sufficient countsfor
9-10 elements.Even if the sampleis analyzedfor Na
firsr the concomitantincreasein other elementscontinuesto occurthroughthe remainingcount-time.
Q) For EDS analysis: use a "thin window" if available,or operatein windowlessmode.This allows more
countsto enterthe detectorand so reducescount-time.
However, windowlessmode risks permanentdamage
to the detectorif the probevacuumfails.
(8) For WDSanalysis:owing to the easewith which
TIIE I{ ITCTRON-MICROPROBE
ANALYSIS
OF GLASS
331
Flc. 6. Data-processingoptions to corect for migration of alkali metals. Performing
pre-ZAF correctionsfor all elementsin order to rectify beam-inducedion mobility
effectsis the only propersolution.
the alkali metals are mobilized in the WDS mode at
routine operating conditions, analyzefor Na and K
first so that their loss or gain is minimized. Note that
this doesnot reducethe obverseeffect (Figs. lc, f) on
other elements(e.9.,Robertset al. 1,990).
(9) Data corrections: Correct all elementsfor bearninduced ion-mobiJity effects prior ta ZAF correction
Gtg. 6). Collect decay or enrichmentcurves for all
elementssimultaneously.Na loss may be estimatedby
exfrapolatingthe count-rateback to zero time (Nielsen
& Sigurdsson1981). Someworkers correct for alkali
loss or gain by adjusting the Na nd K values ajler
on-line ZAF processing,but suchan approachfails to
compensatefor the apparentincrease(or decrease)in
the remaining(unmobilized)elementsand leadsto the
compoundingof analyticalerror.
(I0) Intemartonal standards:use intemational glass
standards(e.9., Myers et al. 1976, Abbey 1983,
N.I.S.T. 1994)to calibrateandto test for alkali migration and checkoverall results.
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